Neurons in laminae I and II of the lumbar spinal cord of the cat were characterized physiologically in terms of responsiveness to various natural stimuli (such as pinch and brush), peripheral input (C fiber vs. A-delta vs. A-beta), and effects of nucleus raphe magnus stimulation. These neurons were then intracellularly injected with horseradish peroxidase, the cats perfused, and the spinal cord sectioned and reacted with diaminobenzidine. The neurons were identified, the tissue processed immunohistochemically with antibodies to serotonin and substance P, and the cells and immunoreactive boutons then drawn by camera lucida technique with high power light microscopy. Major anatomic features, including dendritic branching pattern, dimensions of branch segments, and numbers of contacts on shafts and spines of dendritic segments, were stored in the NIDR PDP 11/70 computer. Programs were written to analyse general anatomic features of subpopulations of neurons.